Method for fabricating a polycrystalline silicon film

ABSTRACT

A method for fabricating polycrystalline silicon film is to add a semitransparent film between a substrate and silicon film. When using the laser to light to the silicon film, the semitransparent film absorbs a portion of laser energy and thus the semitransparent film keeps in high temperature during solidification of silicon film. The silicon film will keep molten for a long time and therefore have more time for crystal grain growth. The crystal grain size of polycrystalline silicon film in this method is much larger than normal substrate heating method.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method of fabricating a polycrystalline silicon film and more particularly to a method that is adapted to apply to the laser annealing process and has a larger crystal grain size of a polycrystalline silicon film than normal.

[0003] 2. Description of Related Art

[0004] The major material of the semiconductor is silicon and there are many forms of the silicon generally used, such as the amorphous, the polycrystalline and the single crystalline. Polycrystalline silicon thin film is lately attracted considerable attention due to its special physical features and low cost, and so companies fabricating thin film transistor liquid crystal displays (TFT-LCD), TFT-Organic Light Emitting Displays (TFT-OLED) or silicon solar cells are interested in developments of this material.

[0005] Polycrystalline silicon is an aggregate of single crystal grains and thus there are many grain boundaries between crystal grains. These grain boundaries are a scattering source of carrier and therefore, enlargement of crystal grain size is very important for a high performance devices. The conventional methods for fabricating the typical polycrystalline silicon film are as follows:

[0006] 1. Solid phase crystallization (SPC).

[0007] 2. Vapor phase deposition (VPD).

[0008] 3. Laser annealing.

[0009] In SPC and VPD method, the crystal grain size is as small as 100 nm and therefore the electrical performance of poly-Si film is bad. In laser annealing method, excimer laser is a most typical light source due to its high light energy density. In excimer laser annealing (ELA )method, an intense light beam with ultra-short duration can heat the precursor silicon film up to its melting point to be crystallized during its solidification process while keeping the non-heat-resistant glass or plastic substrate at sufficiently low temperatures. Therefore good electrical properties are expected in ELA fabricated poly-Si film because of its high process temperature from an angle of Si film. However, the drawback of ELA method is that the crystal grain size is smaller than 0.3 um in a 100 nm-thick silicon film.

[0010] In the ELA method, the silicon film is molten by laser light and then the silicon crystallizes during its solidification process. The longer the melt duration is, the larger crystal grain size. If thermal flow rate from melting silicon to substrate can be suppressed, crystal grain size can be enlarged. There is several ways for limiting the heat to keep in the melting silicon, such as supplying a low specific heat film between silicon film and the substrate (W. Yeh et. al., Jpn. J. Appl. Phys., 38(1999)L110) or heating the substrate to decrease the temperature gradient between the substrate and the silicon film for slowing the heat flow down (H. Kuriyama et. al. Jpn. J. Appl. Phys., 30(1991)3700; J. S. Im et al., Appl. Phys. Lett. 64(1994)2303). Materials with sufficiently low specific heat is porous materials, however, the porous materials is not only difficult to fabricate but also has low alkali-resistant and/or acid-resistant. Therefore using the porous film is usually destroyed in the back end processes for fabrication of devices.

[0011] As per the above description, for improving the silicon film has large crystal grain size in ELA, the heating substrate way seems better for improving the silicon film to have a large crystal grain size. The crystal grain size in ELA with 800° C. substrate heating is two times larger than that with room temperature. However, the heating temperature in substrate heating method is restricted by the strain temperature of substrate materials. For example, 500° C. is the upper limit for glass substrate and 150° C. for plastic substrate, and therefore the effect of substrate heating on crystal grain size under these temperatures is slight.

[0012] To overcome the shortcomings, the present invention provides a method wherein the large crystal grain size of the silicon film is fabricated according to the method to mitigate and obviate the aforementioned problems

SUMMARY OF THE INVENTION

[0013] The main objective of the present invention is to provide method for fabricating a polycrystalline silicon film with big crystal grains.

[0014] Other objects, advantages, and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a lateral view of a silicon film and a substrate in accordance with present invention;

[0016]FIG. 2 is chart showing threshold laser fluence for complete melting of the silicon film versus absorption coefficient of a semitransparent film;

[0017]FIG. 3 is chart showing a crystal grain size versus absorption coefficient of semitransparent film; and

[0018]FIG. 4 is a lateral view of the silicon film and the substrate in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERED EMBODIMENT

[0019] With reference to FIG. 1, a method for fabricating a polycrystalline silicon film on a substrate (1) comprises the following steps:

[0020] applying a semitransparent film (2) on a top surface of the glass substrate (1), wherein a SiOxNyCz compound included N, O, C and Si elements fabricated by a Radio Frequency Plasma Enhanced Chemical Vapor Deposition (RF-PECVD) using Tetramethylsilane, O₂ and N₂, and the semitransparent film (2) has a thickness of 800 nm and has a 0˜40000 cm⁻¹ absorption coefficient for 248˜351 nm wavelength.

[0021] applying a buffer film (3) on the semitransparent film (2) wherein the SiO₂ material is deposited by RF-PECVD on the semitransparent film (2) and is about 100 nm; and

[0022] applying an amorphous silicon film (4) on the buffer film (3) by the Low Pressure Chemical Vapor Deposition (LPCVD) using Si₂H₆ gas at 500° C. and the thickness of the amorphous silicon film (4) is 100 nm; and

[0023] heating the semitransparent film (2) with an absorption coefficient and the silicon (4) to crystallize the silicon film (4) by a laser (5) in room temperature, wherein there are many ways to heat the semitransparent film (2) such as applying an incident single pulse of XeF excimer laser (5) from a backside of substrate (1) into semitransparent film (2) and silicon film (4).

[0024] The absorption coefficient of the semitransparent film (2) was measured from transmittance and reflectance of the semitransparent film on a quartz substrate (1) by spectrophotometer, and the absorption coefficient of quartz substrate (1) in mentioned wavelength is defined as 0 cm⁻¹ here. The crystal grain size was measured by scanning electron microscopy after Secco-etching of silicon film (4).

[0025] With reference to FIG. 2, a relation between the threshold laser fluence for complete melting of the silicon film and absorption coefficient of the semitransparent film is shown. The threshold laser fluence increased with increasing absorption coefficient. Since the energy for melting the silicon film is the same among all samples, the surplus of laser energy should be absorbed by the semitransparent film and be turned into heat. The more absorption coefficient, the more laser energy the semitransparent film absorbed, and therefore, the higher temperature the semitransparent film becomes. When the laser fluence is above 700 mJcm⁻², this is maximum permissible laser fluence for the semitransparent film, the semitransparent film peeled off.

[0026]FIG. 3 shows the crystal grain sizes as a function of absorption coefficients of semitransparent film that has about 0 cm⁻¹ to 100000 cm⁻¹ absorption coefficient. The crystal grain size is defined as the radius of disk structure obtained under a near complete melting condition of the silicon film. The diameter of disk structure in substrate heating method can refer to following issue (J. S. Im et al., Appl. Phys. Lett. 64(1994)2303). The crystal grain size is almost the same for absorption coefficient under 4000 cm⁻¹, but increased dramatically at above 4000 cm⁻¹ with increasing the absorption coefficient, and is maxim at 3.8 μm when absorption coefficient is 12000 cm⁻¹. This value is 10 times larger than the value with absorption coefficient of 0 cm⁻¹, and is even larger than substrate heating method (radius is 0.9 μm at 600° C.) (J. S. Im et al., Appl. Phys. Lett. 64(1994)2303). The tendency in FIG. 3 is also valid for using KrF excimer laser light source with using quartz as a substrate; means there is no dependence on laser wavelength in this method. From FIG. 3 the absorption coefficient need to be over 4000 cm−1.

[0027] As per the above description, when a laser light (5′) with a certain wavelength for which the silicon film (4) becomes semitransparent was used as light source, the laser light (5′) can also be applied from a topside of the silicon film (4) instead of the backside of the substrate (1), as shown in FIG. 4 and FIG. 1. The laser light (5′) energy can also be absorbed by both the silicon film (4) and the semitransparent film (2).

[0028] As per the above description, the semitransparent film peels off when the laser fluence is over 700 mJcm⁻² at absorption coefficient of over 12000 cm⁻¹. The maximum permissible laser fluence for the semitransparent film is related to the resistant temperature of the semitransparent film. Therefore, more heat resistant materials can be applied for the semitransparent film, and if larger laser fluence can be increased by using more heat resistant material, the crystal grain size can be increased furthermore.

[0029] For an embodiment of a material for semitransparent film, SiOxCy is a candidate because of its high thermal stability. Arranging atom ratio between O and C can vary the absorption coefficient.

[0030] For another embodiment of a material for semitransparent film, when the silicon atom occupies more than 33 percent of the SiOx compound, the SiOx compound becomes a film with an absorbency. That is, when the quantity of the silicon atom in the SiOx compound is adjusted, the absorption coefficient of the SiOx compound is changed. Many other materials with heat resistant and appropriate absorption coefficient can be applied as a semitransparent film.

[0031] As per the above description, increasing the maximum laser fluence for semitransparent film lead to further increase of crystal grain size. For an uniform distribution of absorption coefficient of the semitransparent film along depth direction, the temperature will be higher at a side the laser light incident in and lower at a side laser light incident out. In order to increase the maximum laser fluence for the semitransparent film, it is also useful to arrange the absorption coefficient along the depth direction to standardize the temperature along depth direction. For example of FIG. 1, the absorption coefficient of semitransparent film (2) at near substrate (1)can be lower then that at near silicon film (4) to standardize the temperature in semitransparent film.

[0032] As per the above description, the semitransparent film absorbs a portion of laser energy during laser annealing, and thus the semitransparent film keeps in high temperature during crystallization of silicon film. The crystal grain size of silicon film in present method applied under room temperature is much larger than normal substrate heating method. Therefore, with the present method to execute the laser annealing process, the equipment of laser annealing process is less than normal, and non heat-resistant materials such as plastic can also be applied as a substrate.

[0033] Although the present invention has been explained in relation to its preferred embodiment, it is to be understood that many other possible modifications and variations can be made without departing from the spirit and scope of the invention as hereinafter claimed. 

What is claimed is
 1. A method for fabricating polycrystalline silicon film, comprising: applying a semitransparent film on a substrate having a top side and a backside; applying a silicon film on the semitransparent film; and heating the semitransparent film and the silicon film by a laser for crystallizing the silicon film, wherein the semitransparent film with an absorption coefficients absorbs a portion of the light from the laser and transforms into heat and the semitransparent film is semitransparent to following laser light.
 2. The method as claimed in claim1, wherein in the heating the semitransparent film and the silicon film step, the laser passes the backside of the substrate into the semitransparent film and the silicon film.
 3. The method as claimed in claim1, wherein in the heating the semitransparent film and the silicon film step, the laser with a certain wavelength for which the silicon becomes semitransparent and the laser passes the top side of the silicon film into the semitransparent film.
 4. The method as claimed in claim1, wherein the absorption coefficient of the semitransparent film is greater than about 4000 cm⁻¹.
 5. The method as claimed in claim1, wherein the semitransparent film is fabricated by a compound consisting of silicon, nitrogen, oxygen, and carbon.
 6. The method as claimed in claim1, wherein the semitransparent film is fabricated by a compound consisting of silicon, oxygen, and carbon.
 7. The method as claimed in claim1, wherein the semitransparent film is fabricated by a compound consisting of silicon and oxygen, wherein the silicon atom occupies over 33%.
 8. The method as claimed in claim1, wherein the absorption coefficient of the semitransparent film has a distribution along depth direction to the plane of semitransparent film. 